Unless you are very far away in frequency (as in - maybe - high UHF receive, low-band transmit) and have the two antennas separated as well, you are going to have desense. Scanners are generally particularly bad about it.
If you connect both devices to the same antenna, and use a diplexer, then you can certainly get away with "crossband" repeating - one receives on (say) VHF, the other transmits on UHF. Many amateur radio dual-band mobile rigs have this feature built in. The diplexer keeps the (bulk of the) RF energy from the transmitter from affecting the receiver.
If you want to retransmit on the same band as the received signal, then you need either a proper duplexer (note the "u" not "i" - they are much more expensive, although some manufacturers use the "duplexer" name improperly for band-separating diplexers) or you need to separate the antennas several wavelengths apart vertically (for typical FM transmissions).
Also, be careful about what you retransmit. You don't want to just rebroadcast all scanner traffic, the powers-that-be won't like that too much...